RISPERDAL® (risperidone) is an antipsychotic agent belonging to a new chemical class, the benzisoxazole derivative. The chemical designation is 3-[2-[4-(6-fluoro-1,2-benzisoxazol-3 -yl)-1-piperidinyl]ethyl]-6,7,8,9-tetrahydro-2-methyl-4 H-pyrido[1,2-a]pyrimidin-4-one.
U.S. Pat. No. 4,804,663, the contents of which are incorporated by reference, describes a synthesis of risperidone. Risperidone may be prepared by condensation of the following two intermediates, 6-fluoro-3-(4-piperidinyl)-1,2-benzisoxazole (Compound I) and 3-(2-chloroethyl)-6,7,8,9-tetrahydro-2-methyl-4H-pyrido[1,2-a]pyrimidin-4-one (Compound II) in dimethylformamide (DMF) in basic conditions (Na2CO3 or K2CO3) with catalytic amount of potassium iodide (KI). The crude risperidone product (III) is crystallized from a mixture of DMF and isopropanol with an overall yield of 46%.

Polymorphism is the occurrence of different crystalline forms of a single compound and it is a property of some compounds and complexes. Thus, polymorphs are distinct solids sharing the same molecular formula, yet each polymorph may have distinct physical properties. Therefore, a single compound may give rise to a variety of polymorphic forms where each form has different and distinct physical properties, such as different solubility profiles, different melting point temperatures and/or different x-ray diffraction peaks. Since the solubility of each polymorph may vary, identifying the existence of pharmaceutical polymorphs is essential for providing pharmaceuticals with predicable solubility profiles. It is desirable to investigate all solid state forms of a drug, including all polymorphic forms, and to determine the stability, dissolution and flow properties of each polymorphic form. Polymorphic forms of a compound can be distinguished in a laboratory by X-ray diffraction spectroscopy and by other methods such as, infrared spectrometry. For a general review of polymorphs and the pharmaceutical applications of polymorphs see G. M. Wall, Pharm Manuf. 3, 33 (1986); J. K. Haleblian and W. McCrone, J. Pharm. Sci., 58, 911 (1969); and J. K. Haleblian, J. Pharm. Sci., 64, 1269 (1975), all of which are incorporated herein by reference.